The present invention relates to ultrasound devices. In particular, it relates to transducer arrays used in medical ultrasound diagnostic equipment.
Medical ultrasound diagnostic equipment use short pulses of ultrahigh frequency acoustic energy to examine tissues of the body. Ultrasound pulses traveling through the body are reflected and scattered by tissue density and elasticity changes. These reflections are detected and plotted in various ways to provide an image of internal body regions such as organs, etc.
Piezoelectric transducers are used to produce beams of ultrasonic energy and to detect the reflections or echoes. In order to complete the image of the desired bodily region under investigation, the beam is scanned. This can be done automatically using either mechanical or electronic scanning. Electronic scanning includes: switching among elements or portions of elements in a linear array; or angular scanning of the beam in a phased array.
Linear arrays or phased arrays provide an opportunity to control the phasing of each element contributing to the pattern response of the array. This in turn provides some degree of control over the shape of the pattern. Phase control allows a designer to focus the beam and electronically scan it. Because of the electronic control, errors are partially compensated for, and the use of mechanical parts and assemblies can be eliminated.
Prior art examples of acoustic transducer arrays are:
U.S. Pat. No. 3,936,791 wherein a linear array of transducer elements is utilized to provide electronic focusing in the longitudinal plane of the array and mechanical focusing in the plane perpendicular to the longitudinal plane;
U.S. Pat. No. 4,180,790 which relates to an electronically controlled aperture wherein the size of the aperture and the focus of the aperture can be dynamically controlled; and
U.S. Pat. No. 4,334,432, which relates to providing linear and quadratic time delay distributions to an array of acoustic transducers in order to provide scan and focusing capabilities.
One common problem encountered with prior art linear and phased arrays is the presence of grating lobes. These arise as redundant or secondary beams in the pattern of the array. In a linear or phased array the time delay associated with energy transmitted or received by each element of the array is not necessarily the same. A time delay distribution across the aperture of the array results in a main beam being formed in the pattern. Depending on the number of elements of the array, the spacing of the elements and the time delay distribution, the beam width of the main beam and the level of the side lobes can be controlled to a considerable extent. However, if the spacing between elements in the array exceeds one half of a wavelength at the operating frequency of the array, then the presence of unwanted secondary beams called grating lobes, which are not as large as the main beam, but which may exceed the average side lobe level, are automatically formed. Grating lobes occur because of the geometry involved in the transducer array.
Because of grating lobes, errors and noise are introduced into the ultrasound imaging or scanning process. Although the main beam is directed at a particular spot of the target area at a given instant in time, the contributions to the return echo include reflections caused by the grating lobes looking at different spots. Hence, the image is smeared by the several contributions. It is, therefore, highly desirable to reduce or eliminate grating lobes. One manner of doing this is to reduce the spacing between array elements to less than one half wavelength at the operating frequency. However, to maintain the same aperture size as arrays with multiple wavelength spacing, one half wavelength arrays would require additional channels which in turn would result in greater expense. Typical transducer element spacings fall between one half and five wave lengths.